Modern server farms or data centers typically employ a large number of servers to handle processing needs for a variety of application services. Each server handles various operations and requires a certain level of power consumption to maintain these operations. Some of these operations are “mission critical” operations and interruptions to these operations may lead to significant security breach or revenue losses for users associated with these operations.
However, transient fluctuations of an AC power to data centers can be unpredictable. For example, power interruptions may originate in commercial power grids, which typically utilize long transmission lines vulnerable to weather conditions (e.g., storms and flooding), equipment failure, and major switching operations.
Thus, when an input power is interrupted, power supplies in server systems need to maintain output having a specific voltage range within a hold-up time. The hold-up time is the amount of time that the server systems can continue to run without resetting or rebooting during the input power interruption.
In a conventional power supply unit (PSU), there are two converter stages; one is a boost converter and the second is a following converter. The first boost converter converts the input source to a stable, higher voltage source. The higher voltage source charges a primary capacitor that serves as the primary capacitance tank for supplying power during a hold-up time. The following converter stage converts the high voltage to a required low voltage output voltage source for the power system. The energy stored on the primary tank energy capacitor, U=C(dV2)/2, can support a corresponding server system during the hold-up time once the input source is turned off. However, not only does the booster converter reduce the overall efficiency of the PSU but also the primary energy tank capacitor needs large high voltage capacitance which occupies a big part of the limited space in a power supply unit. Further, high voltage capacitance is expensive. Finally, the primary capacitor is inefficient as it may only delivery 30% of its capacitance storage volume for the output resident power requirement.